1. Field of the Invention
This invention relates generally to a device for the collection of solar energy and the production and storage of heat for industrial processes, space heating cooling and other processes where heat of up to approximately 100.degree. C. is required. More particularly, a device to collect and store solar energy comprising a covered solar pond and preferably a non-convecting covered solar pond is disclosed.
2. Description of the Prior Art
Much interest exists in means for collecting and storing solar energy. Of particular interest have been means of producing industrial heat process water. Solar water heating is often ideal for these purposes which require water typically in the 50.degree. C. to 80.degree. C. range.
The term "solar pond" has been used to indicate a device in which water plays a significant role in both the collection and storage of solar energy. Although it is theoretically possible to use other fluids, water because of its abundance and usefulness is generally preferred. Such solar ponds are attractive because they are inexpensive to construct and operate.
Solar ponds generally comprise a large, shallow (1-3 meters in depth) pond filled with water. The water is heated by solar insolation. Solar radiation may be absorbed either directly by the water or indirectly by dark, absorbing surfaces forming the bottom and sides of the solar pond. The water in solar ponds is stationary and does not flow as in flow-through or trickle collectors. The heated water in the solar pond also provides thermal storage which is available to average daily and seasonal variations in solar insolation.
Solar ponds may be of two types. The "shallow solar pond" is generally filled with fresh water which is convecting.
The "non-convecting solar pond" may be of several subtypes. Generally a non-convecting pond is produced by the use of a fluid employing a salt gradient. This type of pond comprises three distinct layers: the surface convecting layer, the non-convecting middle layer and the bottom convecting storage layer. Solar radiation is absorbed both within the pond liquid and at the pond bottom, which is usually dark colored. The bottom convecting storage layer contains the fluid with the highest concentration of salt. The lowest concentration of salt is found in the surface convecting layer. A non-convecting layer comprising a fluid exhibiting a salt or concentration gradient wherein the concentration increases with depth is found therebetween. The mass density gradient created by this salt concentration gradient offsets the thermal density gradient created by the hotter water near the bottom of the pond and prevents thermal convection. The non-convecting salt gradient layer acts both as an insulator for the convecting layer at the bottom and as a thermal storage means. Both external and internal heat exchangers to extract heat from such ponds have been employed.
A variant of the salt gradient pond is the saturated salt gradient pond. Such a pond uses a salt whose solubility increases significantly with increasing temperature. A saturated solution of such a salt is maintained throughout all depths of the pond. The difference in solubility with respect to temperature insures that the solution increases in concentration at increasing depths.
Non-convecting solar ponds may also be produced by the addition of various substances which increase the viscosity of the pond fluid to the point where convection is suppressed. Commercial gelling and thickening agents may be used to produce such ponds. Such ponds have been described in U.S. Pat. No. 4,138,992.
Transparent membranes may be inserted either horizontally or vertically into a pond to suppress convection.
Saltless convecting ponds, lacking a salt gradient layer, have also been employed. These ponds must be protected from thermal loss by transparent covers and/or additional night insulation. Floating microglass beads, inflated multiple plastic film glazings, liquid foam, side and bottom insulation or closable lids have been used for this purpose.
A summary of solar pond technology may be found in "An Overview of Solar Pond Technology" by S. L. Sargent, U.S. Dept. of Energy, SERI Site Office, Golden, Colo. 80401.
Many problems are associated with the above solar ponds. The efficiency of such systems has been estimated to be less than about 15%, and generally only about 8%. Further, such ponds are able to provide reliable and steady supplies of heat only under certain conditions.
Salt gradient ponds exhibit additional problems. Evaporation losses from the surface require the addition of fresh water or solutions of minimal salinity. Maintenance of the salt gradient often requires the removal and disposal of highly saline effluent from the pond bottom. This removal and disposal, together with the possible leakage of the saline solution from the pond, often produces contamination problems. These problems may involve surface contamination or contamination of underground aquifers.
The uncovered nature of such ponds also presents many problems. The pond is susceptible to contamination by wind blown debris. Wind stirring also complicates pond management by creating wave action which mixes the graduated solution, reduces the collection of heat and increases evaporation losses.
Algal and other bacterial growth may also reduce the pond clarity and the collection of solar energy. Other problems arise from outgassing of the pond bottom and leakage of the saline solution into local aquifers. Further, such solar ponds are horizontal collectors, reaching a maximum efficiency in summer and exhibiting a minimum efficiency in winter, rather than providing a constant supply of solar energy.